$m = new MongoDBClient("mongodb://localhost:27017");

     $db = $m->mailchimp;
     $collection = $db->users;
      $aggregate[] = [
    '$group' => [
        '_id' => [
            'email_address' => '',
        ],
    ],
   ];
   $data=$collection1->aggregate($aggregate);

PHP Fatal error:  Uncaught MongoDBDriverExceptionCommandException: a group specification must include an _id in /var/www/html/vendor/mongodb/mongodb/src/Operation/Aggregate.php:263
Stack trace:
#0 /var/www/html/vendor/mongodb/mongodb/src/Operation/Aggregate.php(263): MongoDBDriverServer->executeReadCommand('mailchimp', Object(MongoDBDriverCommand), Array)
#1 /var/www/html/vendor/mongodb/mongodb/src/Collection.php(223): MongoDBOperationAggregate->execute(Object(MongoDBDriverServer))
#2 /var/www/html/checklog.php(59): MongoDBCollection->aggregate(Array)
#3 {main}
  thrown in /var/www/html/vendor/mongodb/mongodb/src/Operation/Aggregate.php on line 263
ubuntu@marketer-box:/var/www/html$ php checklog.php
PHP Notice:  Undefined variable: collection1 in /var/www/html/checklog.php on line 58
PHP Fatal error:  Uncaught Error: Call to a member function aggregate() on null in /var/www/html/checklog.php:58